1. Field of the Invention
The present invention relates to a method of manufacturing an anode unit for a solid electrolytic capacitor and an anode unit. The invention relates, more particularly, to a method of manufacturing a sintered anode unit and a sintered anode unit capable of extremely decreasing a content of oxygen that is included in the sintered pellet and also capable of extremely lowering a leakage current of the capacitor, by using a fine metal powder having a valve action. This is for reducing the size of the capacitor and for improving the capacity of the capacitor.
The present invention also relates to a continuous sintering apparatus capable of taking out a sintered unit into the atmosphere by decreasing the content of oxygen included after sintering at a high temperature and in a high vacuum.
Further, the present invention also relates to a method of manufacturing secondary particles of a valve-action metal powder including a small content of oxygen that is excellent in flowability, compactibility or fillability and handleability at the time of a compacting, by using a metal powder of fine primary particles having a valve action.
2. Description of the Related Art
Conventionally, a sintered anode unit of an electrolytic capacitor has been prepared by press-molding or compacting a fine metal powder having a valve action, such as tantalum or aluminum, into a pellet and then by sintering this pellet at a high temperature and in a high vacuum to form a porous sintered anode unit.
In recent years, along with the trend that semiconductor integrated circuits are manufactured in increasingly fine and highly integrated small structures, personal computers and telephones are also provided as portable units in small and light-weigh structures. Under such circumstances, capacitors that are used in these units have also been required to be in small structures with a larger capacity.
In order to meet these requirements, metal powders of finer primary particle sizes have come to be used as a starting raw material having a valve action. This is for achieving a small structure with a larger capacity, by increasing the porosity of the sintered unit and increasing the surface area of the anode unit. For example, particularly in recent years, there have been used raw metal powders having primary particle sizes of 1.5 to 0.7 xcexcm as an average particle size.
Further, in molding such a fine powder, the following practice is general from the viewpoint of the fillability, flowability, compactibility and handleability of the powder. at the time of the molding or compacting. It is general to use a metal powder that has been granulated to have secondary particles by a thermal cohesion of the powder under a heat treatment at a high temperature and in a high vacuum.
Japanese Patent Application Laid-open No. 2-310301 discloses a method of manufacturing a tantalum powder. According to this method, fine primary particles obtained at a high temperature and in a high vacuum are heat treated for 30 minutes at a high temperature of 1,400 to 1,550xc2x0 C. in a vacuum furnace at a degree of vacuum of 10xe2x88x923 Pa. Then, the thermally cohered powder is crushed lightly, and is deoxidized under the heating at 850xc2x0 C. after adding of magnesium.
However, in general, when a metal powder becomes fine, the surface activity becomes high in proportion to the fineness. Particularly, a metal powder such as tantalum that has a valve action, for example, has its surface activity becoming higher along with the degree of fineness. Among metal powders having a valve action that are used as anode units for solid electrolytic capacitors, the metal powder like tantalum has a strong affinity with oxygen. It is general that when the metal powder of tantalum is used as a sintered unit, it is extremely difficult to decrease the content of oxygen included.
When this sintered unit is used as the anode unit for a solid electrolytic capacitor, for example, the increase in the content of oxygen in this sintered unit leads to a deterioration of the leakage current characteristics of the capacitor. Therefore, it is extremely important to lower the content of oxygen included in the sintered unit in the manufacture of the sintered unit.
Japanese Patent Application Laid-open No. 2-39417 discloses a method of manufacturing an electrolytic capacitor that uses a sintered unit as the anode unit. This sintered unit is prepared by sintering a metal powder having a valve action, at a high temperature and in a high vacuum, by paying attention to the content of oxygen included.
According to the proposal in the above publication, the metal powder as a starting raw material before the sintering has an oxygen density of 2,000 ppm or below. However, this publication does not describe a content of oxygen included after the metal powder has been sintered.
As described above, under the above situation, it is important that a sintered anode unit for a solid electrolytic capacitor is prepared as follows. A tantalum powder having a valve action (secondary powdered particles) is press-molded together with a tantalum line. Then, this molded unit is sintered in a high vacuum at a high temperature to form a porous pellet. This porous pellet must be the one that is not adsorbed physically and chemically by the oxygen in the atmosphere even when the sintered unit is taken into the atmosphere.
According to a conventional manufacturing practice, it is general that after a metal powder has been sintered in a high vacuum at a high temperature, the sintered unit is taken out into the atmosphere while lowering the high temperature of the sintered unit in an inert gas atmosphere. Therefore, according to this practice, a leakage to the vacuum occurs that the atmospheric gas is introduced during this process. Further, the surface of the sintered unit is also brought into contact with oxygen in the atmosphere after the sintered unit is taken out from the vacuum chamber.
As explained above, according to the conventional metal powder of a valve action like tantalum having a high affinity with oxygen, it can be understood that it is extremely difficult to lower the content of oxygen included in the sintered unit because of the contact of the sintered unit with oxygen in the atmosphere. This is against the trend of requirement for finer particles that are to be used for capacitors of a compact structure having a higher capacity.
Therefore, even if a known method of a forced cooling is employed under the inert gas atmosphere, there is the following difficulty. The conventional methods of manufacturing a sintered unit, including the methods described in the above publications, cannot sufficiently achieve a reduction in the content of oxygen included due to the contact with oxygen in the atmosphere and/or a reduction in the affinity of the metal powder and the sintered unit with oxygen. There is no sintering apparatus that can sufficiently meet the above requirements, either.
From the above, it can be said that according to the conventional manufacturing method, there is a trend of increasing the content of oxygen when the metal powder has finer primary particle sizes.
In order to decrease the leakage current by decreasing the content of oxygen, as one of the characteristics of the capacitors, the following manufacturing method is necessary. That is, for a metal powder having a high affinity with oxygen, it. is necessary to provide a manufacturing method that does not increase the content of oxygen included in the metal powder after the sintering as compared with the content of oxygen included before the sintering.
Thus, it is the current situation that there is no sintered anode unit for a solid electrolytic capacitor and there is no manufacturing method that can sufficiently satisfy the above-described requirements. In other words, it has not yet been possible to further decrease particle sizes of the starting raw material to meet the requirement for a more compact and a larger capacity of the capacitors in recent years. Further, it has not yet been possible to cancel a further increase in the affinity with oxygen and an increase in the content of oxygen included in the sintered unit due to the finer particle sizes.
It is an object of the present invention to provide a method of manufacturing a sintered anode unit for a solid electrolytic capacitor and a solid electrolytic capacitor capable of decreasing the content of oxygen that gives a bad influence to a capacitor leakage current to 0.1 ppm-/CV (CV represents a product of an electrostatic capacity and a formation voltage) and capable of extremely improving cost performance of time relating to a manufacturing process, even when a metal powder as a starting material of a valve action has an average primary particle size of 1.5 xcexcm or below and when a sintered unit formed by sintering secondary particles provided by thermally cohering the primary particles in a high vacuum and at a high temperature is used as the anode unit for the solid electrolytic capacitor.
Further, it is another object of the invention to provide a tantalum sintered anode unit for a solid electrolytic capacitor as a sintered unit, by using a fine tantalum metal powder having a primary particle size of 1.5 xcexcm or below based on the above manufacturing method.
Further, it is still another object of the invention to provide a method of manufacturing secondary particles of a valve-action metal powder having a small content of oxygen that are excellent in the flowability, compactibility (or fillability), and handleability of the metal powder at the time of press molding or compacting the metal powder, by thermally cohering the metal powder of a valve action having an average primary particle size of 1.5 xcexcm or below under the heating.
In order to achieve the above objects, according to a first aspect of the present invention, there is provided a method of manufacturing a sintered anode unit for a solid electrolytic capacitor, in which a fine metal powder having a valve action is used of which average primary particle size is, for example, 1.5 xcexcm or below, and a press-molded or compacted pellet of secondary particles is sintered to have a predetermined low content of oxygen, for example, 0.1 ppm/CV or below.
Further, according to another aspect of the invention, there is provided a continuous sintering apparatus that sinters a metal powder under a deoxidized atmosphere and that takes out a sintered unit including a decreased content of oxygen into the atmosphere. The apparatus has a sintering chamber, a natural cooling chamber, and a forced cooling chamber. The apparatus further has a gradual-oxidizing chamber that repeats, after a forced cooling, a cycle processing of returning a vacuum level to a predetermined high primary vacuum degree, leaking the atmospheric gas into vacuum to lower the vacuum degree, and then setting the sintered unit into a vacuum. Thus, by increasing the atmospheric leakage level step by step, the gradual-oxidizing chamber is finally set to the atmospheric pressure, and the sintered unit is taken out.
In other words, according to the method of manufacturing an anode unit for a solid electrolytic capacitor using this continuous sintering apparatus relating to the present invention, the pellet is sintered at a predetermined high temperature and in a predetermined vacuum degree. Then, the pellet is naturally cooled at the same predetermined primary vacuum degree. Thereafter, the pellet is shifted into an inert gas atmosphere, and is cooled compulsively.
Next, the pellet is shifted into a gradual-oxidizing chamber in the same predetermined primary vacuum degree, and a small volume of atmospheric gas is introduced to lower the. vacuum level. The chamber is returned to the predetermined primary vacuum degree again. Thus, the cycle of vacuum, atmospheric leakage and vacuum is repeated. After the leakage level is increased step by step to set the chamber to the atmospheric pressure by this cycle processing, the sintered pellet is taken out from the gradual-oxidizing chamber into the atmosphere.
Further, according to still another aspect of the invention, there is provided a sintered anode unit for a solid electrolytic capacitor using a tantalum metal powder having an average primary particle size of 1.5 xcexcm or below, wherein the sintered anode unit includes a content of oxygen by 0.1 ppm/CV or below.
Further, according to still another aspect of the invention, there is provided a method of manufacturing secondary particles of a valve-action metal powder having particle sizes within a range of 10 to 200 xcexcm and including a low predetermined content (ppm/CV) of oxygen, for example 0.1 ppm/CV or below, by using a fine metal powder of a valve action having primary particle sizes of 1.5 xcexcm or below.
In other words, according to this method, the metal powder is thermally cohered in a predetermined high primary vacuum degree to have secondary particles. Then, the secondary particles are naturally cooled at a predetermined primary vacuum degree. Thereafter, the secondary particles are shifted into an inert gas atmosphere, and are cooled compulsively.
Next, the secondary particles are shifted into a gradual-oxidizing chamber in the same predetermined primary vacuum degree, and a small volume of atmospheric gas is introduced to lower the vacuum level. The chamber is returned to the predetermined primary vacuum degree again. Thus, the cycle of vacuum, atmospheric leakage and vacuum is repeated. After the leakage level is increased step by step to set the chamber to the atmospheric pressure by this cycle processing, the secondary particles are taken out from the gradual-oxidizing chamber into the atmosphere.
According to the present invention, the above-described method of manufacturing secondary particles and the method of manufacturing a sintered anode unit are employed. According to these methods, even when the average primary particle size of a valve-action metal powder is as fine as 1.5 xcexcm or below as a starting raw material of the secondary particles and a porous sintered anode unit, an increase in the oxygen adsorption content can be restricted extremely as follows. After the secondary particles have been thermally cohered or the pellets have been sintered, they are subjected to the repetitive cycle processing of the vacuum, atmospheric leakage and vacuum many times in the gradual-oxidizing chamber of the continuous sintering apparatus. Based on the gradual increasing of the leakage level, heat discharging becomes always larger than the increase in the surface temperature even when the sintered unit and the secondary particles are brought into contact with oxygen during the atmospheric leakage. This is extremely different from the conventional processing method. Therefore, it is possible to extremely restrict the increase in the oxygen adsorption content. As a result, the secondary particles and the sintered unit can be taken out into the atmosphere by preventing heating and natural combustion.
As explained above according to the present invention, there is no risk of heating and combustion due to a natural oxidation, unlike the conventional method. Further, it is possible to extremely restrict the content of oxygen included, and it is possible to extremely reduce the processing time after the forced cooling. As a result, it is possible to extremely improve the time cost performance.
Further, according to the method of manufacturing secondary particles based on the cycle processing of vacuum, atmospheric leakage and vacuum in the gradual-oxidizing chamber, it is possible to remove the conventional deoxidizing processing by heated magnesium after the thermal cohesion. As a result, it is possible to simplify the process of making the secondary particles.